Burning coal to produce electrical power is one of the critical 21st century power generation dilemmas. Fifty-five percent of global power comes from burning coal. The resulting flue gas emissions from burning coal contain a broad spectrum of intractable climate-change and health-compromising compounds. For instance, carbon dioxide, a climate changing gas, is virtually impossible to economically eliminate in power-generating facilities. The use of natural gas instead of coal reduces, but does not eliminate the carbon dioxide. Both coal and natural gas also discharge ozone-producing gasses and soot particulates, which are costly to extract from exhaust and flue gas.
Natural gas infrastructure for domestic space and water heating is fully developed as a preferred fuel source. It is advantageous for ground transportation and as feedstock for a broad range of chemical processing. The available natural gas stores, however, are much more limited than coal. The use of natural gas for electrical power generation appears to be misguided in the long term.
To provide a perspective, the Tennessee Valley Authority (“TVA”) Kingston Fossil Plant, burns about 14,000 tons of coal a day, but produces over 50,000 tons of carbon dioxide per day. Their discharge of ozone and particulate emissions is not stated. This plant powers 700,000 homes, but requires daily delivery of 140 freight-car loads of coal that must be dug out of the ground and transported to Kingston. Coal extraction, cross country delivery, on site handling and burning can be directly related to human costs, and can be directly related to mounting environmental degradation for just this one plant.
About twenty percent of world power is produced from water-cooled nuclear fission with varying degrees of public acceptance, from passive but reluctant acceptance to hysterical fear and absolute demands for nuclear power elimination. Existing nuclear power plants provide inherent risks, but are engineered and operated to exceptional safety standards. Nuclear power plants were originally designed for a 20-year life. An increasing number of nuclear plants are approaching an age of 60 years. Ten- and 20-year operating license extensions have been repeatedly granted after comprehensive examinations and analysis. Satisfying solutions to plant aging are elusive.
Population growth, rising standards of living and economic growth are putting world-wide electrical grid generating capacity margins at risk. Conservation and alternative generating sources are helpful, but are not expected to meet the growing demand. In addition, electrical demand growth to power the growing worldwide demand for air conditioning and the anticipated demand for electric cars are projected to further over stress the capacity of existing grids. Clearly, there is a growing demand for more electrical power, but current methods of power generation are problematic and unsustainable.
In 1824, Sadi Carnot described the ultimate heat engine efficiency limit of a perfect engine dependent on the highest heat-input temperature and the lowest waste-heat rejection temperature. Rankine, Diesel, Otto and Brayton conceived basic power-generation engine cycles and others have refined these basic engines. The Atkinson cycle is a recent improvement of the Otto and Diesel cycles. General Electric (“GE”) and Siemens have developed open gas turbine/steam combined-cycle power plants. Each has made unique contributions to power production technology.
Steam-based Rankine cycle engines dominate electric power generation. A Rankine-cycle engine has two possible energy sources, burning coal or other fossil fuels, and nuclear fission. In both, superheated steam at high pressure drives a turbine that in turn drives an electrical alternator. A steady, continuous, recirculating flow of water and steam flows through a boiler, turbine, condenser and water pump in this closed system. The heat source is external combustion of coal, sub-grade hydrocarbons or natural gas, or from a boiling-water nuclear reactor. Waste heat is rejected from the turbine exhaust at or near ambient dew-point temperatures in a steam-condensing heat exchanger. This low temperature waste heat rejection temperature is key to normal cycle efficiencies in the 35 percent (“%”) to 40% range. However, the continuous, superheated, steam turbine inlet temperature is limited to about 1000 degrees (“°”) Fahrenheit (“F”) to avoid hydrogen embrittlement of the turbine blades. This material limitation precludes higher efficiencies from operating at higher superheated steam temperatures. This superheated steam temperature limit exists for both combustion and nuclear heat sources.
Coal-fired units produce electricity by burning coal in a boiler to heat water to produce steam, generally employing a coal/fossil fueled, closed Rankine cycle (steam) power plant. Steam, at tremendous pressure, flows into a turbine, which spins a generator to produce electricity. The steam is cooled, condensed back into water, and the water is pumped back to the boiler to continue the process.
For example, the coal-fired boilers at TVA's Kingston Fossil Plant near Knoxville, Tenn., heat water to about 1000° F. (540° Celsius (“C”)) to create steam. The steam is piped to turbines at pressures of more than 1,800 pounds per square inch (130 kilograms per square centimeter). The turbines are connected to the generators and spin them at 3600 revolutions per minute to make alternating current electricity at, e.g., 20,000 volts. River water is pumped through tubes in a condenser to cool and condense the steam discharging from the turbines. The Kingston plant generates about 10 billion kilowatt-hours a year, or enough electricity to supply 700,000 homes. As mentioned previously hereinabove, to meet this demand Kingston burns about 14,000 tons of coal day, an amount that would fill 140 railroad cars daily.
The open Brayton cycle is generally used in gas turbine and combined-cycle power plants that burn liquid or gaseous fossil fuels, and produce refractory environmental stressors. The turbine blades and other structures formed of superalloy materials to limit oxidation and creep temperature properties, however, limit turbine operating temperatures to about 2000° to 2100° F. Complex internal turbine blade cooling systems enable turbine inlet gas temperatures to exceed 2500° F., but these high temperatures produce a full range of harmful ozone activators and high levels of nitrous and nitric oxides (“NOx”). Typical turbine exhaust temperatures of 500° to 700° F. compromise efficiency to mid-40 percent range. In conventional fossil-fueled power plants, whether designed for steam or gas turbines, the combustion products are ozone-producing gases, carbon dioxide, and particulate soot that are environmental stressors. These toxic exhaust products cannot be easily eliminated, and are costly to reduce. Climate stability-challenging carbon dioxide removal from coal fired boilers is not practical at this time.
In nuclear powered power-generating plants, high-pressure steam is produced by contact cooling of water with fission-reacting fuel rods. In the heating process, the circulating water and steam become radioactive. This large mass of radioactively contaminated water is an unavoidable and an unfortunate side effect in all existing nuclear power plants. Consequently, all existing nuclear plants must absolutely prevent water and steam venting or leakage. They must also be actively controlled in all operating modes to prevent “melt down” and accompanying water dissociation, hydrogen explosions, and uncontrolled spread of radioactive gases, liquids, and particles. Prevention of these types of failures is a high tribute to comprehensive and exhaustive excellence in engineering, manufacturing, and vigilant operation in a safety culture.
Notwithstanding these precautions, three reactor meltdowns have happened in the past half century including Three Mile Island without injuries. Another incident occurred at Chernobyl with 31 on-site deaths and long-term evacuation of a 1000 square mile region, plus undisclosed, high human and animal sickness and early deaths. In 2011, multiple melt downs at the Japanese Fukushima power plants followed a tsunami with monumental tragedies.
Limitations of conventional power generation approaches have now become substantial hindrances for wide-scale power generation with high efficiency and low levels of undesirable environmental pollutants. No satisfactory strategy has emerged to provide a sustainable, long-term solution for these issues. Accordingly, what is needed in the art is a new approach that overcomes the deficiencies in the current solutions.